The need for alarm systems in the home has become accepted and is reflected by the recent increase in the number of smoke detectors being sold. Other security systems such as heat detectors, gas detectors, and intrusion monitors have also become more commonplace, especially in commercial establishments. However, an earthquake is one hazard from which occupants of a building are largely unprotected. An earthquake alarm system would be useful in that it would warn the occupant of the seismic event so that they could quickly seek protection before the occurence of aftershocks.
Inertia type switches sensitive to motion can be used to detect earthquakes and are quite well known. Many use balls in detents so that upon movement the balls can be vibrated free to complete an electrical circuit. See, for example, U.S. Pat. Nos. 3,733,448; 3,878,858; 3,927,286; and 4,124,841. Another type uses balls which roll in transversely placed tracks so that the balls contact switches at the end of the tracks when the track is caused to tilt. Such a device is shown in U.S. Pat. No. 3,269,685. An inertia type switch having a cylindrical inertia mass which slides over a surface is shown in U.S. Pat. No. 3,779,262.